feat(timezone): add timezone-aware session context (#671)

* feat(timezone): add timezone-aware session context (#661)

All timestamps were UTC-only, causing daily logs to split at UTC midnight,
cron schedules to fire in UTC, and no quiet hours for heartbeat. This adds
timezone as a per-session property flowing from the client.

Key changes:
- New `src/timezone.rs` module with resolution chain, parsing, and detection
- `IncomingMessage` carries optional timezone from client
- `JobContext.user_timezone` flows timezone to tools
- `next_cron_fire()` accepts timezone for schedule evaluation
- `Trigger::Cron` stores optional timezone (backward-compatible)
- Workspace gains `_tz` variants for daily logs and system prompt
- Heartbeat supports quiet hours (`HEARTBEAT_QUIET_START/END`)
- Web frontend sends `Intl.DateTimeFormat().resolvedOptions().timeZone`
- REPL auto-detects system timezone
- `DEFAULT_TIMEZONE` env var / settings for server-wide default

Storage stays UTC. Conversion happens at display boundaries.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address review feedback on timezone-aware sessions

- Validate quiet hours values (0-23) in HeartbeatConfig::resolve()
- Fall back to settings values when env vars are unset for quiet hours
- Validate IANA timezone strings in routine_create/update with parse_timezone
- Add timezone field to routine_create tool schema
- Allow standalone timezone update on cron routines without changing schedule
- Return path from append_daily_log_tz to avoid TOCTOU race at midnight
- Delegate append_daily_log to append_daily_log_tz(entry, UTC) to avoid drift
- Preserve timezone through approval flow via PendingApproval.user_timezone
- Improve test_today_in_tz to not depend on hardcoded year
- Add 3 regression tests for quiet hours config validation

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: fix formatting in routine.rs

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address second round of review feedback

- Remove .claude/scheduled_tasks.lock from repo and add to .gitignore
- Store resolved timezone (not raw message.timezone) in PendingApproval
- Carry forward user_timezone through chained approvals in thread_ops
- Wire quiet_hours_start/end from config to HeartbeatRunner
- Support X-Timezone header as fallback in chat_send_handler

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): include user's local time in time tool response

The time tool's "now" operation now returns local_iso and timezone
fields based on ctx.user_timezone, so the LLM can report time in
the user's timezone instead of always UTC.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: fix formatting in time.rs

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix(timezone): address Copilot review round 3 — validation, deterministic tests, schema fixes

- Validate DEFAULT_TIMEZONE and HEARTBEAT_TIMEZONE at config load time
- Add timezone field to HeartbeatSettings and config::HeartbeatConfig
- Wire heartbeat timezone from config through agent_loop to HeartbeatRunner
- Add timezone to routine_update tool schema (was accepted but not advertised)
- Error on schedule/timezone update for non-cron routines
- Validate timezone in Trigger::from_db (coerce invalid to None with warning)
- Validate timezone in approval path (thread_ops.rs) before overwriting
- Time tool always includes timezone/local_iso fields (fallback to UTC)
- Make quiet hours tests deterministic using current UTC hour
- Add regression tests for config validation

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/context/state.rs

@@ -164,6 +164,8 @@ pub struct JobContext {
     /// previous results by ID via `$tool_call_id` parameter syntax.
     #[serde(skip)]
     pub tool_output_stash: Arc<tokio::sync::RwLock<HashMap<String, String>>>,
+    /// User's preferred timezone (IANA name, e.g. "America/New_York"). Defaults to "UTC".
+    pub user_timezone: String,
 }
 
 impl JobContext {
@@ -203,9 +205,16 @@ impl JobContext {
             http_interceptor: None,
             metadata: serde_json::Value::Null,
             tool_output_stash: Arc::new(tokio::sync::RwLock::new(HashMap::new())),
+            user_timezone: "UTC".to_string(),
         }
     }
 
+    /// Set the user timezone on this context.
+    pub fn with_timezone(mut self, tz: impl Into<String>) -> Self {
+        self.user_timezone = tz.into();
+        self
+    }
+
     /// Transition to a new state.
     pub fn transition_to(
         &mut self,